Summary
- Examine HQ Tau, intermediate mass T Tauri star undergoing magnetospheric accretion.
- Variations in radial velocities (RV) observed in 2019-2020 cannot be solely attributed to stellar spot activity; a binary is one explanation.
- Multi-output Gaussian process regression to model RVs and bisector inverse slopes (BIS).
- Conclude that there is at least one companion; suspected brown dwarf.
Data
- 39 polarimetric observations.
- Collected over six months (2020 - 2021).
- Four instruments:
- SPIRou
- ESPaDOnS
- Neo-Narval, and
- SOPHIE.
- 14 observations in 2017 reported in Pouilly et al. (2020)
Data: Radial Velocities (RV)
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“mean of \(V_r \approx 17\) km s−1 on a period of about 100 days”
Lomb-Scargle \(P \approx 2.5\) days.
Data: Bisector Inverse Slope (BIS)
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Lomb-Scargle \(P \approx 2.42\) days.
Data
Q: Averaging bisectors throws away information?
Methodology
- Obtain high-res spectroscopy.
- Fit MOGP to RV and BIS.
- No companion (without Keplerian modulation)
- One companion (with Keplerian modulation)
- Two companions (with Keplerian modulation)
- Compare the model fits
Used pyaneti package with 500 MCMC chains.
Multi-Output GP Model
- Two response variables: RV, BIS.
- Mean function is Keplerian function (or zero).
- Covariance kernel: white noise (WN) + quasi-periodic (QP).
- Interpretation of hyperaparameters:
- \(P_{GP}\), rotation period
- \(\lambda_e\), stellar spot lifetime
- \(\lambda_p\), “wiggliness” within a period
- \(T0\), transit epoch
- \(K\), semi-amplitude
Results: No companion
Results: One companion
Results: One companion
- Brown dwarf companion with minimal mass \(50.1^{+29.6}_{-32.0}\) Mjup.
- Semi-major axis of \(\approx 0.61\) AU.
- Period of \(126^{+32}_{-55}\) days.
Results
Conclusions
- There is at least one companion to HQ Tau.
- Strongly suspect a brown dwarf plus a third component.
- Need more observations outside the inflexion point of tertiary orbital motion.